Various types of projectiles which can be fired at an animal from a distance and which on impact inject a liquid through a needle into the animal have been proposed. The liquid, e.g., liquid tranquilizer, is typically stored in a cylindrical "primary chamber" within the projectile. One side of a movable piston or the like is typically in contact with the liquid within the primary chamber. The opposite side of the piston faces a "secondary chamber" which includes means for driving the piston toward the primary chamber. Movement of the piston toward the primary chamber pressurizes the liquid and causes it to flow through the needle into the animal.
Unfortunately, most prior art hypodermic projectiles cause considerable damage to animals. They damage outer tissue, including hide, upon impact; then they damage inner tissue layers when they violently dispense their liquid contents.
Several characteristics of prior art hypodermic projectiles contribute to their tendency to cause tissue damage. For example, many existing injecting or hypodermic projectiles include relatively aggressive triggering mechanisms for releasing the liquid from the primary chamber after the projectile strikes its target. For example, some projectiles include a chemical charge in the secondary chamber which explodes upon impact to drive the piston toward the primary chamber. Reference is made to U.S. Pat. No. 3,209,695, issued to Crockford et al. As another example, some projectiles include pressurized secondary chambers, wherein the secondary chamber is pressurized prior to firing of the dart or as a result thereof. See, e.g., U.S. Pat. Nos. 4,103,893, issued to Walker and 3,209,696, issued to Palmer et al. When the needle strikes and penetrates the animal, the primary chamber is placed in fluid communication with the hollow needle and the pressurized secondary chamber causes the piston to rapidly push the liquid through the needle and into the animal.
Experience has shown that there are problems associated with a pressurized secondary chamber, whether the pressure is due to chemical explosion or a compressed gas. For example, it is very difficult to guarantee that the secondary chamber will be properly pressurized in all cases. If the secondary chamber is insufficiently pressurized, the piston separating the primary and secondary chambers will be unable to force the entire contents of the primary chamber through the needle. On the other hand, if the secondary chamber is overly pressurized, the liquid in the primary chamber will rapidly spurt through the needle. This can damage the animal's tissue. In fact, "gas-propelled" pistons can inject liquid into an animal at such an explosive rate that the liquid literally bores a hole in the animal's tissue. The volume of liquid injected (approximately 2 to 4 cc) is large enough to cause considerable trauma to the animal's tissue. Tissue trauma can also be caused simply by the impact of a dart; this problem is discussed below.
In addition to the secondary chamber problem discussed above, it is perceived that prior art injecting projectiles have inadequate barbs. A barb is defined herein as any type of lateral protrusion on the penetrating needle which helps retain the needle within the animal during the liquid injection process.
Barbs are typically prefabricated and attached to dart needles in separate assembly operations. U.S. Pat. Nos. 3,209,695 and 3,209,696 show darts having such prefabricated barbs. While the use of prefabricated barbs can be cost effective, it is perceived that this technique can create problems. For example, the bond between the barb and the needle might fail, in which case the barb will not help retain the needle within the animal. If the tip of the needle does not remain in the animal's muscle for a sufficiently long period of time, some of the liquid could be deposited within the fatty subcutaneous layer immediately beneath the animal's hide or otherwise wasted. And, if the barb should become disconnected from the needle when the needle and barb are in the animal, the barb could remain in the animal when the needle is withdrawn, potentially harming the animal.
Also, a prefabricated barb could be improperly attached to its needle. For example, a prefabricated barb could be attached backwards, such that its biting edge or lip is toward the tip of the needle. A backwards barb could obviously cause unnecessary damage to an animal. In addition, a backwards barb would not assist in retaining the needle in an animal during the injection process.
Aside from the problems associated with bonding the barb to the needle, some barbs are overly "aggressive." An overly aggressive barb can be defined as one which extends laterally from the needle to an excessive extent or which is shaped to hook an animal's hide, making removal difficult. If a barb is too aggressive, it will retain the needle within the animal for an unnecessarily long period of time. The needle need only be within the animal for a period of time sufficient to allow transfer of the contents of the primary chamber into the animal.
In addition, most prior art hypodermic projectiles are so complex as to be too heavy for accurate long range injections and excessively costly. The weight of prior art hypodermic projectiles is particularly troublesome. Prior art projectiles often include aluminum or steel components. Unfortunately, the heavier the dart, the greater the trauma to the animal occasioned simply by the momentum of the dart. Animals which are inoculated, for example, with prior art projectiles almost invariably suffer a hematoma at the dart's entry point.
Thus, while prior art hypodermic projectiles are generally useful for their intended purposes, as a class they possess several shortcomings. In summary, they often include complicated trauma-producing triggering mechanisms, disadvantageous barbs and heavy components which can cause impact damage to the animal.
A projectile which addresses most of the problems discussed above is shown in FIG. 1. This projectile, developed by G. L. van Rooyen, includes a simple compression spring in its secondary chamber and does not rely on a pressurized secondary chamber. In fact, a small hole 46 vents the secondary chamber to the atmosphere The single small vent hole 46 allows the spring to freely compress as the primary chamber is loaded and permits the piston to controllably and fully discharge the contents of the primary chamber upon impact of the dart.
Further with regard to the basic van Rooyen dart shown in FIG. 1, the liquid is initially loaded into the primary chamber through the needle. As the primary chamber fills, the piston moves toward the secondary chamber and the spring compresses. Once the primary chamber is filled, the tip of the needle is capped. Alternatively, the piston is placed in a preselected position so as to establish, for example, a 2 cc primary chamber volume; the primary chamber is filled; the needle is capped; and a compressed spring is positioned behind the piston within the secondary chamber to pressurize the primary chamber. In either event, when the projectile strikes an animal, the needle penetrates the resilient cap and the animal's hide, and the pressurized liquid in the primary chamber flows through the needle into the animal in a controlled manner. This controlled delivery of liquid eliminates the tissue damage associated with high flow and high pressure delivery by gas-powered pistons. In addition, the primary and secondary chambers of the basic van Rooyen dart are made of plastic, resulting in a dart which is lighter than earlier darts. Thus, impact-related tissue damage is reduced.
While the basic van Rooyen dart addresses many of the shortcomings of prior art injecting darts, it is perceived that it can be improved. In particular, the present invention is directed toward improving the barb and the venting design of the basic van Rooyen dart. Preferred embodiments of the dart of the present invention are considerably lighter than the van Rooyen dart, resulting in less impact-related tissue damage. And, one preferred embodiment has a needle which is integral with the hollow body, thus reducing manufacturing costs and increasing reliability.
With regard to the barb, the basic van Rooyen dart (shown in FIG. 1) has a barb which consists of a drop of silver solder 44 on the barrel of the needle. Not only does this involve an expensive and time-consuming process, it is perceived that there are potential problems associated with the difficulty of closely controlling the size, shape and bonding integrity of the solder drop 44. If the drop 44 is too small, the needle can fall out of the animal prior to delivery of the entire contents of the primary chamber. In fact, the delivery of the liquid alone could supply sufficient rearward pressure on the needle to cause it to fall out of the animal's hide if the barb 44 is insufficiently aggressive.
On the other hand, if the barb 44 on the basic van Rooyen dart is too pronounced the needle can remain in the animal for a period of time after the liquid has been injected. Further, if the drop 44 is too large it can effect the flight aerodynamics of the dart due to asymmetrical wind loading and due to the inherent imbalance created by the solder drop 44.
As in the case of prefabricated barbs, discussed above, the solder drop 44 could disconnect from the needle in which case the needle could prematurely fall out of the animal's hide and the solder drop 44 could remain within the animal. The solder drop 44 could fall off the needle at the time of firing; upon impact with the animal; or simply while the dart is being handled prior to firing.
Finally, the solder drop 44 could easily be longitudinally mislocated on the needle. The location and size of a barb 44 affect the dart's balance which in turn affects the flight of the dart. If the barb 44 is too large and too close to the needle's tip, the front end of the dart will be too heavy in comparison to the tail, and the dart will tend to prematurely dive during flight. Conversely, if the barb 44 is too small and too close to the root of the needle, the dart will tend to climb to a surprising degree.
The size and shape of the barb also directly affect the aerodynamics of the dart by virtue of the fact that the barb is mounted near the tip of the needle. For example, if the barb is excessively large, protruding laterally from the barrel of the needle, unnecessary drag will result and the dart's range will be decreased. It is perceived that the solder drop 44 of the basic van Rooyen dart causes an asymmetrical, erratic wind loading on the dart during flight.
The dart of the present invention includes a barb which is substantially symmetrical about the barrel of the needle. In addition, the barb's location, size and weight can be closely controlled. The barb of the present invention is an integral part of the needle itself, and the fabrication of the barb can easily be automated. Thus the location of the barb and its shape and size can be closely controlled.
It is also perceived that the vent design of the basic van Rooyen dart can be improved. If the secondary chamber is not adequately vented, the rate at which the liquid is injected into the animal can be adversely affected. Also, the vacuum in the secondary chamber caused by inadequate venting can prevent the piston from completing its stroke, thereby causing the dart to deliver only a portion of its contents.
The secondary chamber of the basic van Rooyen dart is vented by a single small hole 46 (referring to FIG. 1) drilled or molded through the wall of the secondary chamber. This single hole can easily be plugged during the manufacturing process, for example. Plugging is even more likely in the field, where a dart might be reused: while the needle is probably always cleaned between uses, the body of the dart is not treated with such care. The vent hole 46 of the basic van Rooyen dart is quite accessible, making its plugging more likely.
In addition to the plugging problems discussed above, the vent hole 46 of the basic van Rooyen dart is thought to be excessively small. The small vent hole 46 can limit the flow rate of air into the secondary chamber as the piston moves toward the primary chamber. The rate at which the piston moves and the resulting liquid flow rate can be adversely affected.
In preferred embodiments of the present invention, the single small vent hole 46 of the basic van Rooyen dart is replaced by two or more vent holes. Since it is unlikely that all of the vent holes will be simultaneously plugged, the dart of the present invention will almost assuredly deliver the entire contents of the primary chamber to the animal.
Preferred embodiments of injecting projectiles constructed according to the present invention are also considerably lighter than prior art projectiles, including the basic van Rooyen dart. As discussed above, lighter darts cause less tissue trauma to animals.